I have a rather picky home-theater-PC screen. After upgrading Ubuntu on the HTPC from Precise to Trusty, I just couldn’t get the display to sync. After switching out HDMI cables (which did change things, but not for the better) I figured out that xrandr --output HDMI-0 --mode 1348x752 would bring the display back. (It’s a custom mode line.)

However, I couldn’t make this mode change permanent in /etc/X11/xorg.conf. The nvidia-auto-select mode kept overriding things.

The solution was to add this line to the Monitor section of xorg.conf:
Option "ModeValidation" "HDMI-0: NoPredefinedModes, NoXServerModes, NoVesaModes, NoEdidModes"